As one of the fastest growing sectors of automatic identification procedures (Auto-ID), contactless radio frequency identification (RFID) technology has been broadly applied to enterprise supply chain management and other applications.
A typical RFID system includes a RFID reader or interrogator, a RFID chip and a RFID tag antenna. The RFID chip and the RFID tag antenna are typically packaged together to form an RFID tag. The RFID chip stores data which often includes identifying information. The data may be retrieved by a RFID reader through the RFID tag antenna.
A low cost passive RFID transponder or RFID tag does not include any internal power supply. In such a system, the RFID reader sends both information and power to the RFID tag. The RFID tag receives the power and demodulates the data from the reader. The data stored in the RFID chip varies the impedance of the RFID tag antenna so the backscattering wave can be modulated. Then the reader retrieves the data sent by the tag by demodulation. FIG. 1 shows a passive RFID system. In such a system 1, an RFID reader 2 with antenna 4 communicates data and power 6 to an RFID tag having an RFID chip 18. Data 8 is then communicated from the RFID chip 18 to the RFID reader 2.
One of the most significant challenges is that the RFID tag can not be read when using the commonly used dipole antennas when the tags are attached to a conductive surface. The reason why the dipole type RFID tag can not be read is explained in FIG. 2, which illustrates that the image dipole 5 below the metal surface 7 has an opposite current from the original dipole 3. If the space between the dipole and its image is very small (much less than one wavelength), then the total effective current is equal to zero. Thus the total radiated (or received) field is negligible and therefore, the RFID tag is unable to capture data and power from the reader. This is a significant problem given that in many commercial applications it is desirable to apply the RFID tag to a metal or other type of conductive surface.
Based on the literature, a few metal surface mountable RFID tag antennas have been reported. L. Ukkonen, D. Engels, L. Sydanheimo, M. Kivikoski, “Planar wire-type inverted—F RFID tag antenna mountable on metallic objects”, 2004 IEEE AP-S International Symposium (Monterey, Calif.), Jun. 20-26, 2004, vol. 1, pp. 101-104, proposes a 3-layer antenna design which has a horizontal size 10 cm by 8 cm, thickness 6.455 mm, which has a radiation efficiency 0.6753 and directivity 4.347. Based on this performance, the area and the height are too large. The RFID tag antenna is a multilayer structure built on Teflon, thus making it more expensive to manufacture.
M. Hirvonen, P. Pursula, K. Jaakkola and K. Laukkanen, “Planar inverted-F antenna for radio frequency identification”, Electronic Letters, July 2004, vol. 40, No. 14, pp. 848-850 proposes a two-layer structure by using a traditional planar inverted-F antenna. The resulting antenna has a thickness 3 mm and width 45 mm. The deficiency is that the antenna needs two ground connections. The antenna uses Teflon as the substrate. The two shorting plates to ground are in opposite direction, which increases the cost of the fabrication. The best performance for the reading range is about 5.1 m. For this performance, the resulting antenna is still too wide and too thick.
The prior art also provides for various antennas which are mountable on metal and which use inverted-F antenna structures. Even where such antennas demonstrate reasonable reading ranges, such antennas require relatively expensive substrate material (such as Teflon), complicated fabrication processes, thick substrates (3 mm˜4 mm) and result in RFID tag antennas which occupy relatively large areas. Examples of such antennas include W. Choi, H. W. Son, etc. “An RFID tag using a planar inverted-F antenna capable of being stuck to metallic objects”, ETRI Journal, April, 2006, Vol 28, No. 2, pp. 216-218; H.-W. Son, G.-Y. Choi, and C.-S. Pyo, “Design of wideband RFID tag antenna for metallic surfaces”, Electronic Letters, March 2006, Vol. 42, No. 5, pp. 263-265; J.-S. Kuo, K. L. Wong, “Dual-frequency operation of a planar inverted-L antenna with tapered patch width”, Microwave Opt. Technol. Letters, Vol. 28, No. 2, January 2001, pp. 126-127.
Another example of an RFID tag antenna is from Kansas University. The RFID tag antenna uses about 60 mil thick high density polyethylene, an antenna size of 4 inch by 5.5 inch and can achieve 20 feet reading range. Such an antenna may still be too large in size and more expensive then desired.
U.S. Pat. No. 6,329,915 to Brady et al. discloses a method by using high dielectric constant material to reduce the size of the antenna and the distance between the antenna and the conducting surface. The cost may be relatively high and the reading range relatively low due to the high dielectric material.
U.S. Pat. No. 7,126,479 to Claessens et al. discloses a RFID tag which is attachable to a metal container by using a microstrip antenna. The microstrip antenna has a length of half wavelength, which is larger than what may be desired.
U.S. Pat. No. 6,278,369 to Smith et al. discloses an inverted-F antenna which may be applied to a conductive surface; however, the antenna needs two ground connections and a very thick substrate. U.S. Pat. No. 6,914,562 to Forster discloses a surface insensitive antenna structure.
Thus, various attempts have been made to provide metal surface mountable RFID tag antennas. However, such attempts have resulted in designs which are too large, too expensive to fabricate, too limited with respect to reading range, or in designs that otherwise adversely affect the performance of the RFID tag or its commercial viability. What is needed is an improved metal surface mountable RFID tags.
Therefore, it is a primary object, feature, or advantage of the present invention to improve over the state of the art.
It is a further object, feature, or advantage of the present invention to provide an RFID tag which may be applied to metal or other types of conductive surfaces.
It is a still further object, feature, or advantage of the present invention to provide an RFID tag which is economical to manufacture and use.
One or more of these and/or other objects, features, or advantages of the present invention will be clear from the specification and claims that follow.